Riser and method of installing same

ABSTRACT

A hybrid riser having a lower section ( 6 ) and an upper section ( 10 ), said upper section comprising a flexible pipe, and said lower section comprising a substantially rigid pipe in communication with the flexible pipe, said riser further comprising a buoyancy section ( 16 ) at or in the region of an upper end of said rigid pipe ( 13 ). Said buoyancy section ( 16 ) also comprises an elongate cylindrical buoyancy element, which may be of a coaxial compartmentalised tubular construction having valves such that it may be controllably flooded or evacuated. The hybrid riser may be tethered to a surface vessel or to the seabed. The hybrid riser may be constructed on land, and towed to the vicinity of the installation to which it is to be connected.

The present invention concerns a riser, for the transport of fluid hydrocarbons from an underwater wellhead.

Over recent years, as liquid and gaseous hydrocarbon fuel resources have become more scarce, there has been a tendency to search for and extract these resources in increasingly remote and inaccessible areas, and in particular in increasingly deep parts of the sea. As the depth of these sub-sea wells below sea level has increased, the demands on the technologies used to bring the oil or gas from the well to the sea surface have increased dramatically. As more and more sophisticated techniques for overcoming these difficulties are devised however, the costs and risks associated with the construction, installation and use of these structures has grown.

Risers of two different types are commonly used in bringing oil or gas from a sub-sea well to the surface. Flexible pipes have the advantages of being relatively easy to install, reducing the cost and risks involved in the installation process itself, and being resistant to work fatigue brought on by cyclic forces exerted on the pipe by the movement of the sea and other factors. On the other hand, solid steel pipes are considerably less costly to manufacture.

On the basis of these strengths and limitations, flexible pipes have tended to be chosen for use in shallow installations. This is the case firstly since the total length of the pipeline needed for a well positioned in shallow water is sufficiently short that the cost of the more expensive flexible pipe is not prohibitive.

As well as the two types of piping commonly used to serve as risers, there are furthermore two basic geometrical configurations which are frequently used. The first of these is the vertical riser, where the riser rises directly from the sub-sea well to a vessel on the surface of the sea. The second is a catenary configuration in which the riser starts from the well head running along the seabed, before rising up from the seabed towards a vessel floating or otherwise situated on the sea's surface, such that the riser forms a gentle curve in the shape of a catenary, or the riser catenary extends directly from the well head itself to the support vessel.

FIG. 1 shows the use of flexible piping in a modification of the catenary configuration, as is known in the prior art. In this embodiment, a vessel 3 is disposed on the surface of the sea 1, with a flexible riser 4, extending from the seabed up to the vessel in a configuration known as a “lazy S”. An anchored mid water arch 5 is provided over which the riser 4 passes so as to provide an upper catenary section and a lower catenary section. By means of the “lazy S” configuration movement of the vessel 3 induced by the wind, waves, currents etc., does not result in the flexible riser overbending and rubbing against the ground in such a way as to induce wearing of the riser. This arrangement thus affords protection to the touch-down zone 6, where the riser meets the seabed 2.

FIG. 2 shows an alternative configuration of a flexible riser known from the prior art, which is commonly referred to as a “lazy wave”. In this configuration, the flexible riser rises to the vessel 3 from the seabed 2 and is provided with a plurality of buoyancy elements 7 at a point along the flexible riser's length, such that a portion of the flexible riser is lifted up from the curve which the riser would otherwise adopt under its own weight as it rises from the seabed. A portion of the riser is thus retained substantially stationary in the water by the buoys, and is relatively unaffected by the movement in the vessel 3. The buoyancy elements 7 generally consist of pre-formed foam discs, which are clamped around the flexible riser.

In situations where it is necessary to provide a riser between the sea surface and a wellhead in deep water, the price for the required length of flexible riser becomes less attractive. The use of a rigid riser thus becomes more desirable. FIG. 3 shows a rigid riser configuration. Rigid risers are generally constructed from steel piping, in the form of tubular sections welded together. In the configuration shown in FIG. 3, this is known as a steel catenary riser (SCR). This configuration comprises a continuous steel pipe, which may be several kilometres long rising to a vessel 3 on the sea surface from the seabed 2, bending in a smooth curve, such that the rigid riser leaves the wellhead in a substantially horizontal manner and arrives at the support vessel in a vertical or nearly vertical orientation.

Although relatively cost effective, this configuration has the drawback that variations in the position of the vessel 3 result in cyclic stresses at the touch down point 6 of the catenary riser 9, such that the rigid riser is fatigued over time, and is prone to failure.

A first solution to this problem is known from U.S. Pat. No. 5,639,187, which discloses a configuration such as shown in FIG. 4. According to this configuration, there is disclosed a hybrid configuration, in which one or more risers are draped over a bar 12 which is supported in the water by two buoyant elements 11 a and 11 b, disposed at either end of the bar 12 such that the three elements together form an H shape. This H shaped structure is anchored to the seabed by lines 19 a, 19 b, 19 c and 19 d. The riser between the bar 12 and the seabed 2 comprises a steel catenary riser 13, and the riser between the surface vessel 3 and the bar 12 comprises a flexible riser 10.

Thus, in operation, the riser bar 12 remains substantially stationary in the water, regardless of movements in the surface vessel 3, such that the steel catenary riser 13 is isolated from cyclic effects, and the touch down zone 6 is not fatigued. Further, the load on the surface vessel from the risers is reduced, since the buoy carries the weight of the lower part of the risers. The cost and effort involved in installing and maintaining the system, in particular the anchoring of the float arrangements, is substantial however.

An alternative solution to certain of the problems arising from deep sea wellheads is suggested by the Patent WO 00/53884, which describes a further hybrid arrangement as shown in FIG. 5. According to the teaching of this document, there is provided a riser having three sections. These three sections comprise an upper flexible riser section 10 ascending to the surface vessel 3, a second lower flexible riser section 15 ascending from the seabed, and a rigid riser section 14 disposed between and interconnecting the two flexible riser sections 10 and 15. This configuration has the advantage that substantially all forces resulting from movement of the surface vessel 3 are absorbed by the two flexible portions of the risers 10 and 15.

This arrangement naturally comes with certain of the drawbacks of the fully flexible catenary configuration discussed above however. Further, the load of the risers at the surface vessel is relatively high.

A further alternative arrangement is the tower riser, which comprises a rigid, vertical riser tower provided with air tanks at the upper extremity, and connected to a surface vessel by a flexible riser pipe.

The present invention seeks to overcome drawbacks of various riser configurations discussed above. Objects of the present invention thus include the provision of a riser configuration which is suitable for use in deep water, is less prone to fatigue effects or abrasion, and is of comparatively low cost and simple to construct and install.

According to the invention from a first aspect there is provided a riser having a lower section and an upper section, said upper section comprising a flexible pipe which may be made for example of standard flexible pipe, composite material or titanium and said lower section comprising a substantially rigid pipe in communication with the flexible pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe.

In a second aspect of the invention there is provided a riser having a lower section and an upper section, said upper section comprising a flexible pipe which may be made for example of standard flexible pipe, composite material or titanium and said lower section comprising a substantially rigid pipe in communication with the flexible pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe, said buoyancy section being tethered to a vessel on the sea's surface. This has the advantage that it is less costly to tether a riser to a surface vessel than to the seabed. It had not previously been thought possible to achieve satisfactory stability in the riser by this technique. The inventor has found that recent changes in anchoring techniques have made the required stability achievable.

According to the invention from a third aspect there is provided a riser having a lower section and an upper section, said upper section comprising at least one flexible pipe and said lower section comprising at least one substantially rigid pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe, said buoyancy section comprising an elongate buoyancy unit extending lengthwise of the rigid pipe. This configuration has the advantage of being relatively simple and cost effective to construct and install by a variety of methods as outlined further on.

According to a development of this third aspect of the invention said buoyancy section is tethered to a vessel on the sea's surface.

According to a further development of this third aspect of the invention said buoyancy section is tethered to the seabed.

A further tether may be provided between a point on the rigid pipe above the high bending area thereof, and the seabed.

The lower section may comprise a plurality of rigid pipes and said upper section may each comprise a corresponding plurality of flexible pipes. This construction is advantageous in that it provides for the transportation of different fluids in different directions through the same pipeline requiring a single installation process.

According to a further development, said plurality of rigid pipes may be arranged around the outside of said buoyancy section and may in addition be spaced apart evenly about the circumference of said buoyancy section.

Each of the plurality of rigid pipes may be fixed at or near an upper extremity thereof to said buoyancy section.

According to a further development, said buoyancy section may be further provided with a plurality of sleeves intended to slidingly receive said plurality of rigid pipes respectively.

According to a further development, the riser may further comprise a spacer connected to each of said plurality of rigid pipes, or to each of said plurality of flexible pipes so as to maintain said plurality of rigid pipes or each of said plurality of flexible pipes in a fixed position relative the other rigid pipes or flexible pipes.

According to a further development of any of the above aspects, said spacer is provided at a point on said rigid riser below a lower extremity of said buoyancy section.

According to a development of any of the above aspects each of said at least one flexible pipes is joined to a respective one of at least one rigid pipes at a substantially right angle. This configuration is advantageous in that the connection of a tether to the upper part of the buoyancy section is facilitated.

According to a development of any of the above aspects the substantially right-angled joins between respective rigid and flexible pipes are spaced apart from one another along the length of said buoyancy section. This configuration is advantageous in that the joints between the flexible and rigid sections can easily be separated from one another so as to facilitate connection of these joints during an installation process.

According to a development of any of the above aspects said buoyancy section is made of a foam. This has the advantage of not being affected by leaks in the structure of the riser.

According to a development of any of the above aspects said buoyancy section is a tube arranged such that the rigid riser runs therethrough. This construction is highly cost effective.

As a further development of this construction there is provided a riser wherein said buoyancy section is made of steel, titanium, aluminium or a composite material. A buoyancy unit according to this construction can be produced using the techniques conventionally used in the construction of rigid riser pipes, therefore further improving economy and autonomy.

As a further development of this construction there is provided a riser wherein said solid tube is arranged coaxially to said rigid riser. This configuration makes it easier to ensure a predictable spacing between the inner and outer tubes, with associated effects on the mechanical and thermodynamic properties of the riser as a whole. This is commonly called a Pipe-In-Pipe system

As a further development of the above constructions there is provided a riser wherein said buoyancy section further comprises a plurality of bulkheads dividing said buoyancy section into a plurality of closed chambers. This makes it possible to finely control the buoyancy of the buoyancy unit, and therefore the riser as a whole. It also introduces a degree of leak and damage resistance.

As a further development of this construction there is provided a riser wherein at least one valve is provided allowing flow of a fluid from inside said rigid riser to the interior of a respective at least one of said closed chambers. A further development involves the provision of at least one valve allowing flow of a fluid from inside a respective at least one of said closed chambers to the exterior of said buoyancy unit. By means of these valves it is possible to control the buoyancy of the buoyancy unit.

According to a development of any of the above aspects wherein an upper extremity of said rigid pipe is aligned away from the axis of the part of said riser immediately below said upper extremity. By this means it is possible to optimise the transmission of forces along the riser.

According to the invention from a fourth aspect there is provided a method of installing a riser having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of;

-   -   i. constructing a rigid pipe and the buoyancy section on land,     -   ii. at least partially flooding said buoyancy unit, such that         the buoyancy of the riser is negative,     -   iii. towing said rigid riser section and the buoyancy section         out to sea to the location where the riser is to be installed by         at least a first tug using a first tether,     -   iv. allowing said rigid pipe and the buoyancy section to sink to         the floor of the sea,     -   v. connecting the end of the rigid pipe furthest from the         buoyancy unit at a wellhead or flowline tie in,     -   vi. expelling fluid from the buoyancy unit, such that a buoyancy         force is exerted on the buoyancy section,     -   vii. allowing the buoyancy section to rise towards the surface         of the sea under the guidance of said at least one         installation/surface vessel such that the rigid pipe bends         upwards to form a catenary configuration, and     -   viii. attaching a flexible pipe between said         installation/surface vessel and the upper end of the rigid pipe.

This method is advantageous in that it can be carried out without specially adapted deployment vessels.

According to the invention from a fifth aspect there is provided a method of installing a riser having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of;

-   -   i. constructing a rigid pipe and the buoyancy section on land,     -   ii. weighting said buoyancy unit, such that the buoyancy of the         riser is negative,     -   iii. towing said rigid pipe section and the buoyancy section out         to sea to the location where the riser is to be installed by at         least a first tug using a first tether,     -   iv. allowing said rigid pipe and the buoyancy section to sink to         the floor of the sea,     -   v. connecting the end of the rigid pipe furthest from the         buoyancy unit at a wellhead or flowline tie-in,     -   vi. removing said weighting from the buoyancy unit, such that a         buoyancy force is exerted on the buoyancy section,     -   vii. allowing the buoyancy section to rise towards the surface         of the sea under the guidance of said at least one         installation/surface vessel, such that the rigid pipe bends         upwards to form a catenary configuration, and     -   viii. attaching a flexible pipe between said         installation/surface vessel and the upper end of the rigid pipe.

According to a development of this method one or more temporary buoyancy elements are connected to said rigid riser such that buoyancy is distributed along the length of the riser substantially evenly.

According to a development of this method, the lower end of said rigid riser may be connected to said well head by means of jumpers or rigid spools.

According to a development of the above method, said rigid pipe and the buoyancy section are towed out to sea to the location where the riser is to be installed further using a second tug and a second tether said second tether being connected to a point along the rigid riser behind the point to which said first tether is connected. By this means the riser can be steered and manoeuvred by the second tug while the first tug provides motive force.

According to a development of the above method, said rigid riser is pressurised with a gas prior to said step of expelling fluid from the buoyancy unit. This makes the provision of external pumping or pressurising means during the installation process unnecessary.

According to the invention from a sixth aspect there is provided a method of installing a riser configuration having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of:

-   -   i. constructing the entire riser structure on land,     -   ii. at least partially flooding said buoyancy unit, such that         the buoyancy of the riser is negative,     -   iii. towing the riser out to be installed at sea to the location         where the riser is to be installed by at least a first tug (42)         using a first tether (44),     -   iv. allowing riser to land on the seabed,     -   v. connecting the end of the rigid pipe furthest from the         buoyancy unit (16) at a wellhead or flowline tie-in,     -   vi. expelling fluid from the buoyancy unit, such that a buoyancy         force is exerted on the buoyancy section,     -   vii. allowing the buoyancy section to rise towards the surface         of the sea under the guidance of said at least one         installation/surface vessel, such that the rigid pipe bends         upwards to form a catenary configuration, and     -   viii. attaching a flexible pipe to a surface vessel.

This simplifies the installation process since it is no longer necessary to connect the flexible and rigid riser pipes underwater.

The above methods may also comprise the step of attaching a tether between the buoyancy unit and said surface/installation vessel or the seabed, so as to reduce the strains exerted on the riser.

The above methods may also comprise the step of attaching a tether between a point on the rigid pipe above the high bending area thereof, and the seabed.

According to the invention from a seventh aspect there is provided a method of installing a riser having a lower section and an upper section, said upper section comprising a flexible pipe and said lower section comprising a rigid pipe in communication with the upper section, said riser further comprising a buoyancy section at an upper end of said rigid pipe, said method comprising the steps of

-   -   i. lowering successive lengths of rigid pipe section in the sea,         each length being connected endwise to the length of pipe         section below it;     -   ii. connecting a lower end of a further length of rigid pipe         section having a buoyancy section comprising an elongate         buoyancy unit extending lengthwise of said further length of         rigid pipe section to an upper end of the length of rigid pipe         section immediately below it, to form said rigid pipe;     -   iii. connecting a flexible pipe to an upper end of said rigid         pipe;     -   iv. lowering the flexible pipe;     -   v. allowing the buoyancy unit to sink;     -   vi. adjusting the position of the floating vessel such that the         rigid pipe assumes the configuration of a catenary in the sea         water and,     -   vii. connecting a lower end of the rigid pipe to a wellhead or         flowline.

The buoyancy unit is many times its diameter in length, and is disposed along the length of the upper part of the rigid catenary section.

The buoyancy unit may float freely in the water, or may be tethered to an object on the surface of the sea or at the seabed.

According to the invention from an eighth aspect there is provided a method of installing a riser having a lower section (6) and an upper section (10), said upper section comprising a flexible pipe and said lower section comprising a rigid pipe in communication with the upper section, said riser further comprising a buoyancy section (16) at an upper end of said rigid pipe (13), said method comprising the steps of:

-   -   i. lowering a rigid pipe into the sea by reeling,     -   ii. connecting a lower end of a further length of rigid pipe         section having a buoyancy section comprising an elongate         buoyancy unit extending lengthwise of said further length of         rigid pipe section to an upper end of the length of rigid pipe         section immediately below it, to form said rigid pipe;     -   iii. connecting a flexible pipe to an upper end of said rigid         pipe;     -   iv. lowering the flexible pipe;     -   v. allowing the buoyancy unit to sink;     -   vi. adjusting the position of the floating vessel such that the         rigid pipe assumes the configuration of a catenary in the sea         water and;     -   vii. connecting a lower end of the rigid pipe to a wellhead or         flowline.

According to a development of either of the seventh or eighth aspects of the invention, the buoyancy unit is allowed to sink before said steps of: lowering a rigid pipe into the sea, and connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe.

According to a development of either of the seventh or eighth aspects of the invention the method may comprise the further step of attaching a tether (17 c) between a point on said rigid pipe 13 above the high bending area thereof, and the seabed.

Under a tenth aspect of the invention there is provided a subsea riser comprising a first plurality of service pipes and a further pipe, the first plurality of service pipes and further pipe being in mutually spaced-apart relation, the further pipe being divided along its length into a second plurality of separate sections, each section having a vent valve communicating with the outside of the further pipe, one end of the further pipe having an inlet means for the introduction of a high-pressure gas and the further pipe comprising a third plurality of open-ended tubes extending between respective pairs of adjacent sections. This riser may be employed as part of the riser arrangements described earlier.

For a better understanding of the present invention as well as preferred and other embodiments thereof, reference is made by way of example to FIGS. 1 to 16, in which;

FIG. 1 is a side view showing a riser configuration according to a first arrangement known in the prior art;

FIG. 2 is a corresponding view of a second riser configuration known in the prior art;

FIG. 3 is a side elevation of a third riser configuration known in the prior art;

FIG. 4 is a side view of a riser configuration known in the prior art;

FIG. 5 is a side view of a fifth riser configuration known in the prior art;

FIG. 6 is a side view of a first embodiment according to the present invention;

FIG. 7 a is a side view of a second embodiment according to the present invention;

FIG. 7 b is a side view of a second variant of the second embodiment of the present invention as shown in FIG. 7 a;

FIG. 8 is a side view of a third embodiment of the present invention;

FIG. 9 shows a fourth embodiment of the present invention;

FIG. 10 shows a fifth embodiment of the present invention;

FIG. 11 shows details of the buoyancy unit incorporated in an embodiment of the present invention;

FIG. 12 a shows details of the buoyancy unit 16 incorporated in an embodiment of the present invention in a first state;

FIG. 12 b shows details of the buoyancy unit 16 incorporated in an embodiment of the present invention in a second state;

FIG. 13 shows details of the buoyancy unit 16 incorporated in an embodiment of the present invention;

FIG. 14 shows details of the buoyancy unit 16 and the lower rigid catenary pipe section 13 incorporated in an embodiment of the present invention;

FIG. 15 shows details of the buoyancy unit, the upper portion of the rigid catenary pipe section and the flexible riser of FIG. 9;

FIG. 16 a shows a sixth embodiment of the present invention.

FIG. 16 b shows a cross-section through the diameter of the buoyancy unit (16) of FIG. 16 a through the line AA.

FIG. 16 c shows a cross-section through the diameter of the riser pipes of FIG. 16 a through the line BB FIG. 17 a shows a further development of the sixth embodiment of the invention.

FIG. 17 b shows in further detail the configuration of the elements shown in FIG. 17 a at a cross-section through the line AA.

FIG. 17 c shows in further detail the configuration of the elements shown in FIG. 17 a at a cross-section through the line BB.

FIG. 18 a shows a combination of the embodiments of FIG. 6 and FIG. 17 a.

FIG. 18 b shows in further detail the configuration of the elements shown in FIG. 18 a at a cross-section through the line AA.

FIG. 18 c shows in further detail the configuration of the elements shown in FIG. 18 a at a cross-section through the line BB.

FIGS. 19 a to 19 e show stages of a method of installing a riser of an embodiment of the present invention.

FIGS. 20 a to 20 e shows a further method of installing a riser according to any proceeding embodiment.

FIG. 21 shows the use of a first controlled depth towing method.

FIG. 22 shows the use of a second controlled depth towing method.

FIG. 23 shows the use of a third towing method.

FIG. 24 is a cross-section through a riser arrangement according to a ninth aspect of the invention.

FIG. 25 shows the same arrangement as FIG. 24, but with the section taken directly adjacent a spacer connecting the service riser pipes to a central core pipe, and

FIG. 26 is a diagram showing the creation of buoyancy in the core pipe of FIG. 25.

FIG. 6 shows a first embodiment of the present invention. In this embodiment a hybrid riser ascends to a surface vessel 3 from the seabed 2, and comprises a flexible pipe section 10 connected to said surface vessel 3 and the top part of a rigid riser 13 having the configuration of a catenary, which is further connected to a wellhead on the seabed 2. The surface vessel 3 may be a ship, a semi-submersible unit, a Tension Leg Platform, a Spar platform or other surface vessel as appropriate. The riser may alternatively be terminated at a riser base, and thus not extended all the way to the well head. It may also be used as an export riser. There is furthermore provided a buoyancy unit 16, which is of a substantially elongate shape, such that its length is many times its diameter and is arranged along the length of the upper portion of said rigid riser 13.

Thus, in operation, once installed, the rigid riser portion 13 is held substantially immobile in the sea by the buoyancy unit 16. The flexible riser portion 10 absorbs the motion of the surface vessel 3, and other forces exerted thereon, for example by the movement of the sea itself.

Although FIG. 6 to 10 show the surface vessel 3 as being distant from the well head, it will be appreciated that the arrangement of the present invention also allows for the situation of the surface vessel above the well head. In this, and other cases, the upper riser section will depart from the buoyancy unit at a different location and angle to those shown in FIGS. 6 to 10.

According to a second embodiment of the present invention, as shown in FIG. 7 a, the flexible riser 10 is attached to the upper end of the rigid riser 13 at right angles and hangs suspended in the water in the configuration of a catenary. Furthermore, a tether 17 may be provided between the buoyancy unit 16 and surface vessel 3, or alternatively, a tether 17 b may be provided between the buoyancy unit 16 and a point on the sea surface or seabed.

This arrangement is advantageous over the prior art as embodied for example by the tower riser as discussed above. in that the demands of anchoring the rigid riser to the seabed are reduced, there is no requirement for a flexible joint, and the degree of buoyancy required is reduced.

The positioning of the joint between the rigid and flexible riser parts so that the upper riser part intersects the buoyancy unit 16 at an angle exceeding 45 degrees thereto has the advantage of facilitating the attachment of a tether 17 to the upper end of the buoyancy unit 16 so that extreme movements of the surface vessel 3 will not exert undesirably large forces on the flexible 10, but will rather be absorbed by the tether 17.

According to a preferred realisation of this embodiment, a tether 17 c is connected between the upper part of the lower rigid section 13, and the sea floor. It has been found to be highly advantageous in terms of the stability of the riser, and the limitation of fatigue thereto, to connect the seabed tether 17 c to the riser 13 just over the high bending area, instead of at the buoyancy tank as described with regard to the tether 17 b. It may nonetheless be found desirable to use this configuration in addition to the configuration of tethers 17 b and 17. As an alternative, the tether position 17 c may be transferred to 17 d as shown, i.e. a long way down the riser 13.

A variation of this second embodiment is shown in FIG. 7 b, where the tether 17 is further provided with at least one tensioning weight 18, which causes the tether 17 to deviate from a substantially straight line between the surface vessel 3 and the top of the buoyancy unit 16. A variation of this is to use at least one heavy tether segment such as a chain segment for example.

This has the effect of limiting the forces exerted on the buoyancy unit 16 to a substantially horizontal plane, thereby reducing the risk of inducing fatigue in the lower portion 6 of the rigid riser 13.

According to a third embodiment of the present invention as shown in FIG. 8, the rigid riser section 13 comprises a plurality of individual rigid pipes bundled together. The pipes are intended for carrying the same or different fluids, selected from production fluid, natural gas, injection air and water, for example. Flexible riser sections 10 a, 10 b, 10 c, 10 d, etc. are coupled at right angles to respective rigid pipes in the rigid section 13, at intervals along the parts of the rigid riser section along which extends the buoyancy unit 16. By arranging the perpendicular joints between the rigid and flexible sections in this manner, the connection of the respective rigid and flexible pipes of each pair may be effected without interference from an adjacent pair during installation of the riser structure.

FIG. 9 shows an equivalent structure to that of FIG. 8, but with the further provision of a tether 17 between the top of the buoyancy unit 16 and the sea surface 1.

In operation, the provision of this tether has the effect of reducing the forces exerted on the flexible riser sections 10 a, 10 b, 10 c, 10 d, etc.

According to a preferred realisation of this embodiment, a tether 17 c is connected between the upper part of the lower rigid section 13, and the sea floor. It has been found to be highly advantageous in terms of the stability of the riser, and the limitation of fatigue thereto, to connect the seabed tether 17 c to the riser 13 just over the high bending area, instead of at the buoyancy tank as described with regard to the tether 17 b. It may nonetheless be found desirable to use this configuration in addition to the configuration of tethers 17 b and 17.

According to a fifth embodiment of the present invention shown in FIG. 10, it may be desirable to pre-form the top of the rigid section, such that a top section of the rigid riser extends beyond the upper end of the buoyancy unit and deviates from the line defined by the essentially vertically rising section of the rigid riser.

Alternatively it may be desirable to pre-form the top of the rigid section 13 and the buoyancy unit 16, such that a top section of these two elements 16 a deviates from the line defined by the essentially vertically rising section of the rigid riser.

In operation this may be found to be advantageous, in that the shear forces exerted on the joint between the flexible riser 10 and the rigid riser 13 present when the buoyancy unit 16 is not vertically below the surface vessel 3 are reduced.

FIG. 11 shows further detail of the buoyancy unit 16 used in FIG. 6 (or in any of FIGS. 7 to 10 with obvious adaptation).

In this embodiment, the buoyancy unit comprises an essentially tubular structure arranged coaxially with the rigid riser 13. The buoyancy unit is made of any suitable buoyant material, for example a foam. It may alternatively comprise a rigid hollow buoyant tank made of steel, composite material, aluminium or other materials as will readily occur to the skilled person, which is either an intrinsic part of the upper section of the rigid riser, or a separate tubular structure which is secured thereto. Such a tank may be filled with gas, or a foam or other buoyant material.

FIG. 12 a shows the structure of the buoyancy unit 16 according to a preferred configuration. In this arrangement, the buoyancy unit 16 is formed by positioning the rigid riser 13 coaxially inside a second pipe of larger diameter, such that a tubular space is formed between the two pipes. Six annular bulkheads are provided in this tubular space, so as to divide it into five separate tanks. Furthermore, valves 22 a and 22 b are provided between the inside of said rigid riser 13, and the inside of a second and fourth of said tubular spaces 21 a and 21 b. A further two valves 23 a and 23 b are provided between said second and fourth tubular spaces 21 a and 21 b, and the outside of said buoyancy unit 16.

In operation, the ends of the rigid riser 13 are sealed, and the rigid riser is pre-pressurised, for example with nitrogen (N₂) gas. First, third and fifth tubular spaces 20 a, 20 b and 20 c are similarly filled with gas. The second and fourth tubular spaces 21 a and 21 b are filled with seawater, or another fluid having a higher density than the gas with which the rigid riser pipe 13 is filled. The buoyancy tank compartments may be filled with a light-weight fluid during tow out, which makes the buoyancy section close to neutrally buoyant. This fluid will be replaced by gas (compressed nitrogen or similar) during (or prior to or after) the upending operation (i.e. when the buoyancy tank is elevated into its final position). The gas is supplied from a surface vessel, or from compressed gas in the pipelines, or from compressed gas in the buoyancy tank.

Naturally, the number and configuration of the tanks into which the tubular space is divided may be varied as expedient. It may also be found advantageous to provide tubing connecting the different valves, or to connect one or more chambers together by tubing such that they can be vented through a common valve, or other arrangements so as to facilitate the transfer of fluids to and from the buoyancy unit. It may further be found to be advantageous to provide means such that the transfer of fluids to and from the buoyancy unit can be effected after the installation of the riser.

FIG. 12 b shows the riser 16 of the same preferred embodiment of FIG. 12 a, in a second state. In operation, the valves 22 a, 22 b, 23 a and 23 b can be opened, such that the water in the second and fourth tubular spaces 21 a and 21 b is expelled through valves 23 a and 23 b, and displaced by the pressurised gas from said rigid riser 13, which flows into said second and fourth tubular spaces through said valves 22 a and 22 b.

This has the effect of reducing the overall average density of the buoyancy unit 16.

FIG. 13 shows a further configuration of the structure of the buoyancy unit 16, in which a tubular gap 32 is provided between the rigid riser 13 and the buoyancy unit 16. This gap 32 is defined by the inner wall 33 of the buoyancy unit 16 and the outer wall 31 of the rigid riser 16.

This gap is provided to provide insulation between the fluids flowing through the rigid riser 13 and the surrounding seawater. The gap may be filled with a gas, a fluid or any other insulating material, and may further comprise spacers to maintain the coaxial configuration of the riser 13 and the buoyancy unit 16.

It is also possible to collect a plurality of riser pipes such as that shown in FIG. 13 so as to form a bundle, where each pipe has its own buoyancy sleeve. Naturally, the number and configuration of the riser pipes may be varied as expedient. For example, the rigid pipes may be arranged such that the bundle describes a circle in cross section, or a “flat pack” arrangement, in which the separate pipes are arranged in rows, or other arrangements as may be found to be advantageous.

FIG. 14 shows details of a further design for the buoyancy unit 16 and the rigid riser 13, in which the structure of the buoyancy unit 16 is extended along the entire length of the rigid riser 13. A lower portion of the buoyancy unit 16 is reduced in outer diameter, such that the external diameter of the structure is reduced, the internal diameter of the rigid riser 13 remaining constant. The cross-sectional area of insulating material which may for example comprise gas, foam, gel etc, and may be different in the buoy section than in the riser within the buoyancy unit 16 in this lower section is thus reduced. Thus, in operation, while the upper portion of the buoyancy unit 16 provides substantially all the buoyancy required to support the rigid riser 13, the lower section of the buoyancy unit 16 extends along the length of the rigid riser 13 so as to perform an insulating function.

It is also possible to collect a plurality of riser pipes such as that shown in FIG. 14 so as to form a bundle, where each pipe has its own buoyancy sleeve. Naturally, the number and configuration of the riser pipes may be varied as expedient. For example, the rigid pipes may be arranged such that the bundle describes a circle in cross section, or a “flat pack” arrangement, in which the separate pipes are arranged in rows, or other arrangements as may be found to be advantageous.

FIG. 15 shows further details of the buoyancy unit 16 according to the embodiment of the present invention shown in FIG. 9. According to this embodiment, the rigid riser 13 comprises a bundle of rigid pipes, which are connected at their upper ends to respective flexible pipes perpendicular thereto, at lateral positions spaced longitudinally along the upper portion of the buoyancy unit 16. In the configuration shown in FIG. 15, there are provided six rigid riser pipes, which are connected respectively to flexible riser pipes 10 a, 10 b, 10 c, 10 d, 10 e and 10 f. There is further provided a tether 17, connected to an upper surface of said buoyancy section 16.

Naturally, the number and configuration of the riser pipes may be varied as expedient. For example, the rigid pipes may be arranged such that the bundle describes a circle in cross section, or a “flat pack” arrangement, in which the separate pipes are arranged in rows, or other arrangements as may be found to be advantageous.

FIG. 16 shows a sixth embodiment of the present invention. In this embodiment, two hybrid risers ascend to a surface vessel (3) from the seabed (2), each comprising a flexible pipe section (101, 102) which may be made for example of standard flexible pipe, composite material or titanium composite or other material, connected to said surface vessel (3), and the top part of a rigid riser (131, 132) having the configuration of a catenary, and a lower extremity thereof may further be connected to a wellhead on the seabed (2). The surface vessel (3) may be a ship, a semi-submersible unit, a tension leg platform, a spar platform or other surface vessel as appropriate. The riser may alternatively be terminated at a riser base, and thus not extend all the way to the wellhead. Such a riser may also be used as an export riser or indeed in any other such application. There is further provided a buoyancy unit (16), disposed between the upper part (131) of the first rigid riser, and the upper part (132) of the second rigid riser. The buoyancy unit (16) is of a substantially elongate shape, such that its length is many times its diameter and is arranged along the length of the upper part (131) and (132) of the rigid risers respectively.

The buoyancy unit is made of any suitable buoyant material, for example a foam. It may alternatively comprise a rigid hollow buoyant tank made of steel, composite material, aluminium or other materials as will readily occur to the skilled person, which is either an intrinsic part of the upper section of the rigid riser, or a separate tubular structure which is secured thereto. Such a tank may be filled with gas, or a foam or other buoyant material.

The upper part of each rigid riser (131, 132) may be connected to the buoyancy unit (16) by means of a hang-off arrangement or other bearing or fixture (251, 252), or may simply be welded or otherwise fixed thereto. A similar arrangement may be provided at other points along the length of the buoyancy unit (16). Furthermore, one or more sleeves (261, 262) may be attached to the buoyancy unit, with the rigid riser (131, 132) fitting slidingly through the sleeve. In this configuration, there is provided no permanent connection between the inside surface of the sleeve and the outer surface of the rigid riser, such that the mutual alignment of the buoyancy unit (16) and the rigid riser (131, 132) can be maintained, without exerting any substantial tensional force along the length of the riser pipe.

The mutual spacing of the riser pipes (131, 132) can be maintained below the lower extremity of the buoyancy unit (16) by means of a spacer (29) situated between two clamps (271, 272), each clamp connecting to the rigid riser (131, 132) respectively. The clamps may connect the riser fixedly or slidingly, so as to allow for expansion. By this means, the two rigid risers (131, 132) can be retained in a parallel or other desired configuration as they descend in a catenary manner to the sea floor. It may be desirable to employ spacers of different lengths along the length of the risers, so that the separation thereof changes as a function of distance from the sea bed.

Although as described above with reference to FIG. 16, the two risers are provided on opposite sides of the buoyancy unit (16), that is, separated by an angle of 180°, it is also possible to position the risers at other angles, so as to bring the rigid risers closer to each other on one side of the buoyancy unit (16).

FIG. 16 b shows a cross-section through the diameter of the buoyancy unit (16) through the line AA. In this figure can be seen more clearly the configuration of the rigid riser pipes (131, 132), the sleeves (261, 262), and the buoyancy unit (16).

Similarly, FIG. 16 c shows a cross-section through the diameter of the riser pipes (131, 132) through the line BB, in which can be seen more clearly the configuration of the clamps (271, 272), the risers (131, 132) and the spacer bar (29).

It is also possible to construct the riser structure using conventional methods whilst at sea such as J-lay, reeling etc. For example, by adding pipe sections to the riser one by one as the pipe is deployed from a surface vessel. It may be appropriate to land the buoyancy tank 16 on the seabed and start reeling from there.

Furthermore, the invention is not limited to two riser pipes, but is further applicable to a larger plurality thereof. For example, FIG. 17 a shows a further development of the sixth embodiment of the invention, wherein four riser pipes (131, 132, 133, 134) are disposed around a buoyancy unit (16). As described with regard to FIG. 16 a, the risers may be maintained in position by hang-off units (251, 252, 253 and 254), and sleeves (261, 262, 263, 264) respectively. Furthermore, the risers can be retained in a desired configuration below the lower extremity of the buoyancy unit (16) by means of clamps (271, 272, 273, 274), held in position by a spacer element (29).

FIGS. 17 b and 17 c show cross-sections through the lines AA and BB respectively and show in further detail the configuration of the elements shown in FIG. 17 a.

As will be clear to the skilled person, it is possible to combine the teachings of the above-described embodiments. For example, FIG. 18 a shows a combination of the embodiments of FIG. 6 and FIG. 17, in which a first riser is arranged concentrically with a buoyancy unit (16), such that the riser is surrounded by the buoyancy unit, while a further four risers (131, 132, 133 and 134) are arranged around the outside of the buoyancy unit (16). The external riser pipes (131, 132, 133 and 134) may be maintained in position by hang-off elements (251, 252, 253 and 254) and sleeves (261, 262, 263 and 264) as described in relation to FIG. 17.

All five risers (13, 131, 132, 133 and 134) can be retained in a desired configuration, for example one similar to that imposed by the arrangement of the risers through and around the buoyancy unit (16) respectively, by means of clamps (271, 272, 273 and 274). The central riser 13 may be replaced by a plurality of risers.

FIGS. 18 b and 18 c show cross-sections through the lines AA and BB respectively and show in further detail the configuration of the elements shown in FIG. 18 a.

FIGS. 19 a to 19 e show a method of installing a riser according to any one of the preceding embodiments.

FIG. 19 a shows a first step in this method of installing the riser, in which the rigid riser section 13 which may be typically in the order of seven kilometres long, and the buoyancy section 16 are constructed on land. The rigid riser section and the buoyancy section 16 are then towed out to sea by a tug 42 using a tether 44, according to a procedure sometimes known as a “bundle tow”.

FIG. 19 b shows a second step in the method of installing the riser. In this step the rigid riser section 13 and buoyancy unit 16 are towed out to sea by at least one tug 42 and tether 44, and optionally a second tug 43 and tether 45. The buoyancy unit 16 is preferably partially flooded, such that the riser as a whole has substantially neutral buoyancy. Preferably, this is realised according to the buoyancy unit of FIG. 12 of the present invention, whereby annular spaces 21 a and 21 b are flooded with water, and the rigid riser 13 is pre-pressurised with nitrogen gas.

FIG. 19 c shows a third step of the method of installing the riser. Once the riser has been towed to the vicinity of the well 41, the riser is allowed to sink to or land on the seabed and the end of the rigid riser 13 furthest from the buoyancy unit 16 is connected to the wellhead 41, for example by means of jumpers.

FIG. 19 d shows a fourth step in the method of installing the riser according to the present invention. In this step, the water flooding the ring-shaped spaces 21 a and 21 b is expelled by opening the valves 22 a and 22 b, such that the average density of the buoyancy section 16 decreases, and a buoyancy force is exerted on the buoyancy section 16 towards the surface of the sea. Under the guidance of a tug, or surface vessel 3 by means of a tether 44′, which may or may not be the same tether as used to tow the riser into position, the buoyancy section is allowed to rise towards the surface of the sea, causing the rigid riser 13 to bend upwards to form a catenary configuration.

FIG. 19 e shows a fifth and final step in the method of installing the riser. In this step, a tether 17 is attached to a surface vessel or installation vessel 3, and flexible riser pipes 10 a, 10 b, 10 c, 10 d, etc. are attached between said surface vessel 3 and the upper end of the rigid riser 13, as discussed above with reference to FIG. 9.

The installation of the riser is thus complete, and hydrocarbon transport may commence.

The skilled man will appreciate that variations may be made to the sequence in which the above steps are carried out. In particular, step d, where the buoyancy section is allowed to rise towards the surface of the sea, causing the rigid riser 13 to bend upwards to form a catenary configuration, may in fact take place before the end of the rigid riser 13 furthest from the buoyancy unit 16 is connected to the wellhead 41.

As a variation of the method described above with reference to FIG. 16 a, it is also possible to cause the riser to sink to the sea bed at the installation site by weighting it by means other than allowing the buoyancy tanks to flood. For example chains may be attached to the pipe, such that the riser sinks until the chain drapes on the seabed.

It is also possible to construct the riser structure using conventional methods whilst at sea such as J-lay, reeling etc. For example, by adding pipe sections to the riser one by one as the pipe is deployed from a surface vessel. In the case of reeling it may be appropriate to land the buoyancy tank 16 on the seabed and start reeling from there. More specifically, in accordance with such a constructional method successive lengths of rigid pipe section in the sea, each length being connected endwise to the length of pipe section below it. Then a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section is connected to an upper end of the length of rigid pipe section immediately below it, to form the rigid pipe. A flexible pipe is connected to an upper end of the rigid pipe, after which the flexible pipe is lowered, until the rigid pipe hangs suspended in the water from its upper end. Next the buoyancy unit is allowed to sink and a lower end of the rigid pipe is connected to a wellhead. Finally, to complete the installation, the position of the floating vessel is adjusted such that the rigid pipe assumes the configuration of a catenary in the seawater.

FIGS. 20 a to 20 e show a further method of installing a riser according to any proceeding embodiment.

FIG. 20 a shows a first step in this method of installing the riser, in which the buoyancy section 16 is constructed on land. The buoyancy section 16 Is transported to the installation site on a vessels deck or on cantilever beams outside ship-side or is alternatively towed out to sea by a tug 42 using a tether 44, according to a procedure similar to that sometimes known as bundle tow.

FIG. 20 b shows a second step in the method of installing the riser. In this step, the buoyancy unit 16 is towed out to sea by at least one tug 42 to tether 44, and optionally a second tug 43 and tether 45. The buoyancy unit 16 is preferably partially flooded, so as to have substantially neutral buoyancy. Preferably, this is realised according to buoyancy unit of FIG. 12 of the present invention, whereby annular spaces 21 a and 21 b are flooded with water.

FIG. 20 c shows a third step in the method of installing the riser. Once the buoyancy tank 16 has been towed to the vicinity of the well 41, the buoyancy tank is allowed to sink to or land on the seabed possibly by partially flooding the buoyancy tank such that is buoyancy decreases. According to one realisation of this method, one end of the upper, flexible pipe 10 may be connected to the buoyancy tank 16 prior to allowing the buoyancy unit to sink to the sea floor, so that the connection operation can be performed in relatively shallow water.

FIG. 20 d shows a fourth step in the method of installing the riser according to this embodiment of the present invention. In this step, a cable connected between the buoyancy unit 16, which is eventually to be connected to the lower rigid riser 13, is connected to an installation surface vessel 31 or another surface vessel. A installation surface vessel 31 then proceeds to lower rigid pipe according to a known method, for example J-lay, S-lay or reeling. The rigid pipe 13 is lowered so as to eventually connect to the end of the buoyancy stein therefor. During this process, the tether 45, or other tether connecting a surface vessel with the end of the buoyancy unit 16 intended for connection to the rigid riser may be used to guide the end of the rigid pipe 13, so as to correctly come into contact with the end of the buoyancy unit intended therefor.

FIG. 20 e shows a fifth step in the method of installing the riser according to this embodiment of the invention. In this step, the lower extremity of the rigid pipe 13 has come into connection with the end of the buoyancy unit 16 to which it is to be connected. The connection between these two elements is then made, for example by means of clamps, which may be controlled for example by remote control from the surface. Once again, the tether 45 or other tether connecting the end of the buoyancy unit 16 intended to be connected to the rigid pipe, and a surface vessel can also be used at this stage for insuring the proper relative position of the buoyancy unit 16 and the rigid pipe 13, for the correct functioning of the clamps.

The opposite extremity of the rigid pipe 13, that is the end of this pipe furthest from the end of this pipe connected to the buoyancy unit 16, is released, and lowered to the seabed, where it is connected to the well head 41 or to another pipeline termination point, in a conventional manner. At this stage, the buoyancy of the buoyancy unit 16 can be increased, for example by evacuating sea water from the flooded tanks 21 a and 21 b, so that the buoyancy unit 16 floats towards the sea surface, as described with reference to FIGS. 19 d and 19 e above.

The upper, flexible pipe 10 may now be deployed in any conventional method, and connected to the upper end of the buoyancy unit 16 as described above. This process may be assisted by using the tether 44 as a guide.

The installation of the riser is thus complete, and hydrocarbon transport may commence.

The skilled man will appreciate that variations may be made to the sequence in which the above steps are carried out. In particular, the step where the buoyancy section is allowed to rise toward the surface of the sea, causing the rigid riser to bend upwards to form a catenary configuration may in fact take place before the end of the rigid riser 13 furthest from the buoyancy unit 16 is connected to the well head 41 or to another pipeline termination point.

According to an alternative method, it is also possible to construct the entire riser structure comprising at least one flexible pipe 10, at least one corresponding rigid pipe 13 connected thereto as described above and the buoyancy unit 16 disposed along and the lengthways of the rigid pipe nearest said flexible pipe on land, before towing the whole structure out to be installed at sea.

In all of the embodiments discussed in relation to FIGS. 6 to 19, and in particular those embodiments where there is provided no tethering means, as shown in, for example, FIGS. 6, 8 and 10, it may be found desirable to add weighting to the flexible riser pipe 10. This has the effect of reducing its movement of the buoyancy unit in the water.

In all of the embodiments discussed in relation to FIGS. 6 to 19, the buoyancy unit 16 preferably has a length equal to at least twice its diameter. More preferably, the buoyancy unit 16 has a length equal to at least thirty times its diameter. Yet more preferably, the buoyancy unit 16 has a length equal to at least 100 times its diameter.

In any method of installing the riser of the present invention that involves towing all or part of the riser through the sea, the towed part may be towed using any conventional towing method. For example,

FIG. 21 shows the use of a first controlled depth towing method. According to this method, a plurality of chains 301 is attached to the pipe 13,16 being towed. The pipe is preferably weighted, possibly by being partially flooded, so as to have slightly positive buoyancy on its own, and slightly negative buoyancy when weighted down by the chains. The chains 301 have two effects. Firstly, whilst the pipe is in motion, the chains generate lift, which, when the movement of the pipe through the water exceeds a given velocity, leads to the pipe “flying” in the water. When the tow rate decreases, the pipe sinks gently back towards the sea floor 2. When this occurs, the hanging chains come into contact with the sea floor before the body of the pipe 13 itself. The part of the chain touching the sea floor no longer contributes to the overall weight of the apparatus, so that the pipe ceases to sing as equilibrium is reached between the positive buoyancy of the pipe, and the weight of the chains, so that the pipe remains suspended at a given height above the sea bed.

FIG. 22 shows the use of a second controlled depth towing method. This method has common features with the method described above with regard to FIG. 21. However, according to this method, the lift is provided by aqua foils 302. If required, the pipe is weighted, possibly by being partially flooded, so as to have slightly negative buoyancy on its own, which is counteracted by the lift generated by the aquafoils 302 as the apparatus moves through the water. By balancing the lift from the aquafoils with the weight of the pipe, the attitude of the pipe in the water can be controlled. It is further possible to dispose of one or even both of the tugs 42 and 43, by fitting the riser with thrusters at one or both ends of the pipe. In the case of the riser not being connected to a tug at all, it can be provided with control means allowing it to proceed to the installation point under its own power.

FIG. 23 shows the use of a third towing method. According to this method, the pipe is simply dragged along the sea floor. The pipe is preferably provided with a sleigh or runners 303 to as to reduce the drag of the pipe along the sea floor, and to protect the pipe from wear.

Any of the above three methods is appropriate for use in the installation methods of the present invention.

A further riser arrangement is now described with reference to FIGS. 24-26.

In this arrangement a number of service pipes (risers) 300 (each preferably, but not necessarily, rigid) are collected together into a bundle spaced preferably equidistantly from a further pipe (central core) 302 and held in spatially fixed relationship thereto by clamps 304 (see FIGS. 24 and 25) provided periodically along the length of the core. Preferably, but not necessarily, each riser is provided with its own foam (e.g. “PU foam”) buoyancy material 306. The core is not a service conduit, but a structural member for interconnecting the seabed foundation, to which the risers are to be connected, and a subsea flotation device which is used to tension the risers vertically

Apart from any use of a separate foam buoyancy material 306 for the individual risers 300, it is necessary to provide further buoyancy for the whole riser arrangement. A conventional way of achieving this in the type of riser arrangement just described would be to employ a so-called “syntactic” foam material 308, shown in cross-section in FIG. 24. This material is conventionally of constant cross-section along the length of (or part of the length of) the core which it surrounds. Since this material is expensive, there is an incentive to try to dispense with it, or at least reduce its volume if at all possible.

A way of achieving the above-mentioned aim will now be described, which allows the foam to be dispensed with completely (though, as mentioned, some supplementary foam can also be used, if desired). In this arrangement the core 302 is designed in the manner illustrated in FIG. 26. In FIG. 26 the core 302 is shown in four different stages of its operation when installed in a nominally vertical attitude below sea (though it may assume a tilted attitude in certain applications). The core is divided into a number of individual sections or compartments 310-315 and bridging each adjacent pair of compartments is a tube 320-325. Also provided is a valve 330-335 near the bottom of each compartment.

As regards the operation of the riser arrangement, it is firstly assumed that the arrangement illustrated in FIGS. 24 and 25 has already been installed in the field, the various compartments have been filled with a liquid medium and all the valves 330-335 are closed. The liquid medium may be seawater, fresh water, a gel or even a glass-sphere mix. Corrosion inhibitors may also be introduced, where appropriate. The next step then is to introduce a gas at high pressure into the bottom end of the core, following which the bottom valve 335 is opened. Since the pressure of the gas is higher than that of the surrounding seawater, the liquid medium is forced out of the bottom compartment into the sea via the bottom valve 335. The liquid level in the bottom compartment 315 drops until it reaches to just below the lower open end of the bottom tube 325, upon which the gas enters the tube and passes into compartment 314. At this point the liquid in compartment 314 starts to exit from the valve 334. The liquid level in compartment 314 drops until it eventually reaches the lower open end of tube 324, upon which the gas passes along this tube and into compartment 313, and so on. Once the top compartment has been filled with gas the gas supply is removed.

When the liquid medium in any compartment has reached its minimum level, the valve associated with that compartment is closed. The pressure of the gas in that compartment has the same value as that of the surrounding water at that particular depth and therefore the pressure in each gas-filled compartment counterbalances the hydrostatic pressure of the water at the relevant depth. This means that the core need not be designed to withstand the maximum surrounding hydrostatic pressure, but a lesser value of pressure resistance can be tolerated, thereby saving costs in core materials. (As a rough figure, the core can be designed for typically less than 20% of the fully hydrostatic pressure). The cost reduction can take the form of a reduction in the thickness of the core wall and/or a downgrading of the core material. In addition, of course, there is the original saving in costs brought about by the elimination of the need to use a syntactic foam.

Where maximum buoyancy is required from the core, all the valves 330-335 will be opened at the appropriate time, as outlined above, in turn from the bottom to the top of the core. On the other hand, where only partial buoyancy is required, as soon as a sufficient number of compartments have been filled (or, more accurately, almost filled, due to the pressure of the residual liquid level) with gas, the valve associated with the compartment above the compartment which has last been filled remains closed, as do all the valves higher up the core.

As regards the installation of this riser arrangement, the arrangement is first fabricated on shore and then towed to the operating field in a horizontal attitude by means of tugs. At the field, the riser arrangement is up-ended and connected to the seabed foundation, after which flexible jumpers are attached to the upper ends of the risers 300.

While FIG. 26 shows the use of six risers in the bundle, more or less than six may be included, depending on requirements. Also, it is not a requirement that the risers, however many are used, be equidistantly spaced from each other or from the core. Indeed, the riser pipes may be spaced apart in a line with the “core” (i.e. “further pipe”), with the further pipe situated either adjacent one of the riser pipes on the same line or displaced from the line. In this case, of course, the further pipe cannot strictly speaking be called a “core”.

The riser arrangement shown in FIGS. 24-25 can be employed as part of the riser described earlier and as illustrated in, for example, FIG. 18. In this case the service pipes would represent the risers 131-134 and the further pipe (core pipe) would represent the riser 13. The buoyancy material 16 could be dispensed with or employed in reduced form. As an alternative, the riser 13 could be dispensed with and the buoyancy element 16 realised as a cascade core as shown in FIG. 26.

Although the embodiment so far described has assumed that the various valves will start off in a closed state, it is possible to arrange for them to remain open throughout the whole venting procedure just described. In this case, before the start of venting the liquid medium inside each compartment will not leak out due to the hydrostatic pressure exerted by the surrounding seawater. Also, during the venting process once a particular compartment has filled with high-pressure gas and the gas has been diverted to the next higher compartment, the gas will not escape to the surrounding environment due to the equal pressure outside and inside that compartment. Furthermore, once the core has reached its desired degree of buoyancy, i.e. a sufficient number of compartments have been gas-filled, the gas supply can be simply shut off at that point so that no further compartments are vented. Again, the valves associated with those further compartments can remain open.

In practice, however, it is preferred for the valves to be closed after the venting of their respective compartments. This is because, where the valves remain open, any appreciable vertical movement of the core could cause a transfer of fluid into/out of the compartments from/to the surrounding environment due to the finite pressure differences created by such movement, and this is considered to be undesirable.

Other ways of controlling the valves than have been described may be employed, while still allowing the desired degree of buoyancy to be created in the core.

The reduction in core wall-thickness which this invention allows can result in increased buoyancy solely by virtue of a decrease in weight in the core. However, buoyancy can also be enhanced by converting some of reduction in wall thickness to an increase in core diameter, which creates a higher internal core volume and hence an increase in buoyancy when gas is introduced. The advantage of this is that the individual supplementary foam material 306 provided for each service riser can be even further reduced in volume or even dispensed with altogether. The same applies to any foam that would normally be used to surround the core.

Although the above embodiments of the invention have been described in terms of a production riser connecting a surface vessel to a well head, the skilled person will realise that they are equally appropriate to other applications, for example tying a surface vessel in to a sea floor flowline, so as to function for example as an export riser. It will also be understood that the present invention is applicable to all types of riser. In particular, the present invention applies to water or gas injection risers. 

1-55. (canceled)
 56. A riser having a lower section and an upper section, said upper section comprising a flexible pipe and said lower section comprising a substantially rigid pipe and forming a catenary in communication with the flexible pipe, characterized in that said riser further comprises an elongate buoyancy section fitted around said rigid pipe at or in the region of an upper end thereof.
 57. A riser according to claim 56 further comprising a tether.
 58. A riser according to claim 57 wherein said buoyancy section is tethered by said tether to a vessel on the sea's surface.
 59. A riser according to claim 57, wherein said buoyancy section is tethered by said tether to the seabed.
 60. A riser according to any of claims 57 to 59 wherein a further tether is provided between a point on said rigid pipe above the high bending area thereof, and the seabed.
 61. A riser according to any of claims 56 to 59 wherein said lower section comprises a plurality of rigid pipes and said upper sections each comprise a corresponding plurality of flexible pipes.
 62. A riser according to claim 61 wherein said plurality of rigid pipes are arranged around the outside of said buoyancy section.
 63. A riser according to claim 62 wherein said plurality of rigid pipes are spaced apart evenly about the circumference of said buoyancy section.
 64. A riser according to claim 62 wherein each of said plurality of rigid pipes is fixed at or near an upper extremity thereof to said buoyancy section.
 65. A riser according to claim 62, wherein said buoyancy section is further provided with a plurality of sleeves intended to slidingly receive said plurality of rigid pipes respectively.
 66. A riser according to claim 61, further comprising a spacer connected to each of said plurality of rigid pipes, or to each of said plurality of flexible pipes so as to maintain said plurality of rigid pipes or each of said plurality pipes in a fixed position relative to the other rigid pipes or flexible pipes.
 67. A riser according to claim 66, wherein said spacer is provided at a point on said rigid riser below a lower extremity of said buoyancy section.
 68. A riser according to claim 61 wherein each of said at least one flexible pipes is joined to a respective one of at least one rigid pipes at a substantially right angle.
 69. A riser according to claim 68 wherein the substantially right-angled joins between respective rigid and flexible pipes are spaced apart from one another along the length of said buoyancy section.
 70. A riser according to any of claims 56 to 59, wherein said buoyancy section is made of a foam.
 71. A riser according to any of claims 56 to 59, wherein said buoyancy section is a tube arranged such that the rigid pipe runs therethrough.
 72. A riser according to any of claims 56 to 59, wherein said buoyancy section is made of one or more of: steel, titanium, aluminium and a composite material.
 73. A riser according to any of claims 56 to 59, wherein said lower riser section, or parts thereof, is made of one or more of: steel, titanium, aluminium and a composite material.
 74. A riser according to claim 71, wherein said tube arranged is arranged coaxially to said rigid pipe.
 75. A riser according to any of claims 56 to 59, wherein said buoyancy section further comprises a plurality of bulkheads dividing said buoyancy section into a plurality of closed chambers.
 76. A riser according to claim 75 wherein at least one valve is provided allowing flow of a fluid from inside said rigid pipe to the interior of a respective at least one of said closed chambers.
 77. A riser according to claim 75, wherein at least one valve is provided allowing flow of a fluid from inside a respective at least one of said closed chambers to the exterior of said buoyancy section.
 78. A riser according to any of claims 56 to 59, wherein an upper extremity of said rigid pipe is aligned away from the axis of the part of said riser immediately below said upper extremity.
 79. A riser according to any of claims 56 to 59 wherein said buoyancy section is divided along its length into a second plurality of separate sections, each section having a vent valve communicating with the outside of the buoyancy section, one end of the buoyancy section having an inlet means for the introduction of a high-pressure gas and the buoyancy section comprising a third plurality of open-ended tubes extending between respective pairs of adjacent sections.
 80. A riser according to claim 79, wherein the inlet means is at a lower end of the buoyancy section and the vent valves are disposed near correspondingly lower ends of their respective sections, each tube having a correspondingly lower end situated in a first of its pair of adjacent sections and aligned approximately with the vent valve associated with that first section and a correspondingly upper end situated in a second of its pair of adjacent sections and aligned approximately with the vent associated with that second section.
 81. A riser according to claim 80, wherein the third plurality is equal to the second plurality, one of the tubes having a lower end which is located in the topmost section of the further pipe, and an upper end which communicates with the outside of the further pipe.
 82. A riser according to claim 81, further comprising a control means connected to the vent valves and to the gas inlet and arranged to open each valve in turn starting with the lowest valve, the valve associated with each succeeding section being opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.
 83. A riser according to claim 79, wherein the rigid pipes are arranged around the buoyancy section in an approximately circular configuration.
 84. A riser according to claim 79, wherein each rigid pipe is provided with its own supplementary buoyancy means.
 85. A riser according to claim 79, wherein the buoyancy section is provided with its own supplementary buoyancy means.
 86. A riser according to claim 85, wherein the supplementary buoyancy means is constituted by a foam cladding surrounding the respective rigid pipe and/or buoyancy section.
 87. A riser according to any of claims 56 to 59, wherein the control means is arranged to, in sequence: (a) close all the valves when the core is in a state of minimum buoyancy; (b) apply the gas supply to the gas inlet; (c) open the lowermost vent valve nearest the gas inlet, thereby to allow a liquid medium present in the section nearest the gas inlet to leave that section by way of said valve; (d) open the next valve up when gas starts to be introduced into the next section up; (e) repeat step (d) until the desired or a maximum degree of buoyancy has been attained; (f) stop the gas supply.
 88. A riser according to claim 87, wherein the control means is configured so that the valve associated with each succeeding section is opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.
 89. A method of installing a riser, said method comprising; i. constructing a rigid pipe and a buoyancy section on land, ii. weighting said buoyancy section, such that the buoyancy of the riser is negative, iii. towing said rigid pipe section and the buoyancy section out to sea to the location where the riser is to be installed by at least a first tug using a first tether, iv. allowing said rigid pipe and the buoyancy section to land on seabed, v. connecting the end of the rigid pipe furthest from the buoyancy section at a wellhead or flowline tie-in, vi. removing said weighting from the buoyancy section, such that a buoyancy force is exerted on the buoyancy section, vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel, such that the rigid pipe bends upwards to form a catenary configuration.
 90. The method of claim 89, wherein said step of weighing involves at least partially flooding said buoyancy section, and said step of removing said weighting involves expelling fluid from the buoyancy section.
 91. The method of claim 89 or 90, wherein one or more temporary buoyancy elements are connected to said rigid riser such that buoyancy is distributed along the length of the riser substantially evenly.
 92. The method of claim 89 or 90, wherein the lower end of said rigid pipe is connected to said well head or flowline by means of jumpers or spools.
 93. The method of claim 92, wherein a manifold, wellhead, riser base or further subsea structure is connected to the rigid pipe and towed to field together with the riser in one operation.
 94. The method of claim 89 or 90, wherein said rigid pipe and the buoyancy section are towed out to sea to the location where the riser is to be installed further using a second tug and a second tether, said second tether being connected to a point along the rigid pipe behind the point to which said first tether is connected.
 95. The method of claim 89 or 90, wherein said rigid pipe is pressurized with a gas prior to said step of expelling fluid from the buoyancy unit.
 96. The method of claim 89 or 90 comprising the further step of a flexible pipe between said installation/surface vessel and the upper end of the rigid pipe.
 97. The method of claim 89 or 90 comprising the further step of constructing the entire riser structure on land.
 98. The method of any of claims 79 to 88, further comprising the step of attaching a tether between said buoyancy unit and said surface/installation vessel or the seabed.
 99. The method of claim 79 or 89, further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed.
 100. A method of installing a riser, said method comprising: i. lowering a rigid pipe into the sea by reeling or by lowering successive lengths of rigid pipe section into the sea, each length being connected endwise to the length of pipe section below it; ii. connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy section extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe; iii. connecting a flexible pipe to an upper end of said rigid pipe; iv. lowering the flexible pipe; v. allowing the buoyancy unit to sink; vi. adjusting the position of the floating vessel such that the rigid pipe assumes the configuration of a catenary in the sea water and; vii. connecting a lower end of the rigid pipe to a wellhead or flowline.
 101. The method of claim 100, wherein said buoyancy unit is allowed to sink before said steps of: lowering a rigid pipe into the sea; and connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe.
 102. The method of claim 100 or 101, further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed. 